Setting the EXT-X-TARGETDURATION tag validator to be less strict to be in line with the HLS Pantos Specification

[dcoufal]

Description

This PR fixes a bug where the EXT-X-TARGETDURATION tag validator is too strict. If the EXT-X-TARGETDURATION tag is set to "6", and the fragments are "6.006" this is actually correct from the HLS spec (section 4.3.3.1, https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#page-21).

The previous code would have failed this.

Change Notes

• Changed EXT_X_TARGETDURATIONLengthValidator to implement new rules.
• Added unit tests for new rules.

Pre-submission Checklist

• [x] I ran the unit tests locally before checking in.
• [x] I made sure there were no compiler warnings before checking in.
• [x] I have written useful documentation for all public code.
• [x] I have written unit tests for this new feature.